A powder coating is generally applied on a substrate by using a fluidized bed dip coating, an electrostatic coating, a powdering, a flame spray coating, a flock coating and the like. Such powder coatings may be based on such resins as plasticized PVC, polyethylenes, nylon, polyesters, cellulosics, epoxy resins, chlorinated polyethers, polyurethanes, fluorine, resins and the like, which are selected so as to be optimum for the selected application means.
Among them, vinyl resin is excellent in chemical resistance, abrasion resistance and electric properties and moreover, is less expensive and hence, is widely used in practice. Also, there has been widely accepted a combination of an epoxy resin and a hardener as such a dicyandiamide, a aromatic amine, amine a complex, an acid anhydride, a phenol resin/acid catalyst and the like, because the resulting coating has excellent chemical resistance, toughness, adhesion properties, weather resistance, thermal resistance, abrasion resistance and electric properties.
Such powder coatings are in general far superior to solvent type coating compositions in respect to hygienic and air-pollution characteristics and therefore are becoming more and more popular in the coating area. However, since there remain problems in achieving coating smoothness, gloss, image clarity and the like, they are in general not suitable as a top coat for automobiles and the like and there is a need for the improvement of such properties.
To these ends, various studies have been made. For example, in Japanese Patent Publication (unexamined) No. 864/81 and Japanese Patent Publication 120666/84, there has been proposed a thermosetting type powder coating comprising a vinyl copolymer composed of glycidyl groups containing vinyl monomers and phosphoric acid ester vinyl monomers and having a particular softening point and a specific number average molecular weight, in combination with an aliphatic dibasic acid hardener; in Japanese Patent Publication (unexamined) No. 47456/81, a powder coating comprising, as main ingredients, a phosphor containing vinyl polymer having crosslinkable functional groups and having a softening point of 70.degree. to 150.degree. C. and a number average molecular weight of 2000 to 30000, and a hardener; and in Japanese Patent Publication (unexamined) No. 87471/82, a powder coating has been proposed comprising a thermosetting type resinous composition containing 0.01 to 10% by weight of phosphor-containing polymeric additive.
However, all of these publications rely upon the use of a phosphor-containing binder resin or phosphor-containing polymeric additive, and do not satisfactorily achieve the intended objects and effects. There are also problems in respect to public hygiene and health, environmental air-pollution and the like because of the presence of the phosphor component. Therefore, there is always a demand for powder coatings which do not rely on the use of a phosphor-containing binder resin or additive, and which can be used as a top coat for automobile body and the like.
It is therefore, an object of the invention to provide a powder coating which is free from phosphor-containing polymer and is useful as a top coat for automobile body and the like, and which is also capable of producing a cured coating with excellent smoothness, gloss, image quality, weather resistance, and the like.